1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive optical film and a production method thereof. The present invention further relates to an image display such as a liquid crystal display and an organic electroluminescence (EL) display, including the pressure-sensitive adhesive optical film. The optical film may be a polarizing plate, a retardation plate, an optical compensation film, a brightness enhancement film, a laminate thereof, or the like.
2. Description of the Related Art
The image-forming system of liquid crystal displays or the like requires polarizing elements to be placed on both sides of a liquid crystal cell, and generally polarizing plates are attached thereto. Besides polarizing plates, a variety of optical elements have been used for liquid crystal panels to improve display quality. For example, there are used retardation plates for prevention of discoloration, viewing angle expansion films for improvement of the viewing angle of liquid crystal displays, and brightness enhancement films for enhancement of the contrast of displays. These films are generically called optical films.
When the optical films are attached to a liquid crystal cell, pressure-sensitive adhesives are generally used. Bonding between an optical film and a liquid crystal cell or between optical films is generally performed with a pressure-sensitive adhesive in order to reduce optical loss. In such a case, a pressure-sensitive adhesive optical film including an optical film and a pressure-sensitive adhesive layer previously formed on one side of the optical film is generally used, because it has some advantages such as no need for a drying process to fix the optical film.
Since the optical film used in the pressure-sensitive adhesive optical film can easily shrink or expand under heating or humidifying conditions, the pressure-sensitive adhesive optical film can easily separate or peel after it is attached to a liquid crystal cell. Therefore, the pressure-sensitive adhesive layer is required to have durability against heating, humidifying and so on. The pressure-sensitive adhesive layer is also required to have workability such that it can be worked without pressure-sensitive adhesive fouling or dropout after it is formed on an optical film.
For the durability, a crosslinking agent is added to the pressure-sensitive adhesive so that a crosslinked pressure-sensitive adhesive layer can be formed. The crosslinking agent used is typically an isocyanate crosslinking agent or an epoxy crosslinking agent. When these crosslinking agents are used, however, crosslinking is still incomplete after coating and drying, and, therefore, an aging time is necessary after the coating and drying. The necessary aging time is generally from about one day to about one week. On the other hand, if a cutting process is performed without the aging, the pressure-sensitive adhesive can adhere to a cutting blade, or troubles such as adhesion between laminated upper and lower cut pieces can occur, so that the yield of the manufacturing process can be reduced. In addition, distribution of products is stopped during the aging, which is unfavorable in terms of delivery. If heat treatment is performed to accelerate the aging, the optical film can undesirably undergo changes in size or degradation in optical properties.
In addition, the pressure-sensitive adhesive layer is required to have requisite characteristics. In some cases, for example, if in the process of adhering an optical film to a liquid crystal cell, they are misaligned or foreign matter is caught on the adhering surface, the optical film should be peeled off from a liquid crystal panel so that the liquid crystal cell can be recycled, and the pressure-sensitive adhesive should have re-peelability (reworkability) such that the optical film can be easily peeled off from the liquid crystal panel with no adhesive residue in a peeling process. Particularly in recent years, thin liquid crystal panels having chemically-etched glass plates are frequently used together with conventional panel manufacturing processes, and it has become difficult to realize reworkability or workability of optical films from the thin liquid crystal panels. Further, in some cases, a pressure-sensitive adhesive optical film is adhered to a liquid crystal cell, and then the pressure-sensitive adhesive layer is subjected to a heating and pressure-bonding treatment (such as autoclaving) or a heating and drying treatment for several hours to several tens of hours. In such cases, peeling strength can be very high in the process of peeling off the optical film from the liquid crystal panel so that the gap in the liquid crystal cell can be changed, a part of the optical film can be broken and left on the liquid crystal panel, or the like can occur, and, therefore, the re-peelability (reworkability) is not sufficiently achieved.
In conventional techniques, acrylic pressure-sensitive adhesives are typically used to form pressure-sensitive adhesive layers for pressure-sensitive adhesive optical films. Pressure-sensitive adhesives layers produced from such acrylic pressure-sensitive adhesives generally have a crosslinked structure in which a (meth)acrylic polymer used as a base polymer is crosslinked by a crosslinking agent. For example, it is proposed that the acrylic pressure-sensitive adhesive should include a curing agent and a (meth)acrylic copolymer of divinylbenzene and an acrylic monomer (See JP-A No. 08-218048). It is disclosed that such an acrylic pressure-sensitive adhesive can reduce changes in cohesion and adhesion under high-temperature and high-humidity conditions over time, provide good adhesion particularly between an optical film and a glass substrate, and have high durability such that the acrylic pressure-sensitive adhesive does not cause defects such as peeling off and foaming of the adhesive. It is also disclosed that the acrylic pressure-sensitive adhesive particularly shows an excellent effect, after it is applied to a base material and then irradiated with an electron beam. However, it cannot be said that the conventional pressure-sensitive adhesive optical film can satisfy reworkability, although it can satisfy durability.
Besides the above, it is also proposed that an acrylic pressure-sensitive adhesive for use in bonding optical films to glass materials should include a mixture in which a peroxide is added to a (meth)acrylic polymer (See JP-A No. 2006-183022). It is disclosed that this acrylic pressure-sensitive adhesive can have high durability and good workability (handleability) in terms of manufacturing process based on a thermal decomposition reaction of the peroxide. However, the acrylic pressure-sensitive adhesive described in JP-A No. 2006-183022 cannot achieve satisfactory reworkability.